Interactions among Factors Affecting Stillbirths in Holstein Cattle in the United States

¹ Department of Animal Sciencen, Iowa State University, Ames 50011-3150
² Department of Statistics, Iowa State University, Ames 50011-3150

Each year about 7% of the Holstein calves born in the United States die within 48 h of birth. The exact cause of death is unknown. The purpose of this article is to examine the complex interactions among factors (e.g., parity, season of birth, dystocia, year) contributing to stillbirth rates. A modified chi-squared automated interaction detection algorithm was used to develop classification trees explaining the most likely sequence of factors that result in a stillborn calf. The data were 666,341 births from the MidStates Dairy Records Processing Center and the National Association of Animal Breeders.

Primiparous and multiparous cows clearly differ in the rate of stillbirths, 11.0 and 5.7%, respectively. Dystocia followed parity as the next most important factor within both primiparous and multiparous cows. In primiparous cows, season, year of birth, or gestation length ranked third as an important predictor for dystocia equal to 1, 2, or 3+, respectively. Gestation length ranked third in importance among the factors that affect stillbirth rates for all levels of dystocia in multiparous cows. Among multiparous cows needing assistance (dystocia 3+), stillbirth rates were greatest for shorter gestations less than the average of 280 d, 55.3% for –15 to –12 d, 45.5% for –11 to –9 d, 33.7% for –8 to –5 d, 23.8% for –4 to 13 d, and 35.4% for 14 to 15 d. Gestation length pinpointed the time when stillbirths occurred, as indicated by the increase from 23.8% stillbirth rate among calves born at or above the mean gestation length to 55.3% for those calves born –15 to –12 d below the mean gestation. Further investigation of the relationship between stillbirth rates and gestation length is needed to develop a more complete understanding of the biological processes resulting in the loss of calves at birth.

Primiparous cows and multiparous cows were best analyzed separately. In multiparous cows, the third most influential factor was gestation length at all levels of dystocia. Among births needing assistance, gestation lengths of –4 to 13 d had similar stillbirths rates of 23.8%. The analysis procedure also defined three intervals below the mean with significantly different stillbirth rates; –15 to –12 d (55.3%), –11 to –9 d (45.0%), and –8 to –5 d (33.7%) This suggest that gestation length did not have a linear, quadratic, or exponential trend in relation to stillbirths.

Key Words: chi-squared automated interaction detector • dystocia • Holstein • stillbirth

Submitted on November 16, 1999
Accepted on May 22, 2000

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